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The photolysis of acetone in the presence of triethylborane
Jownui of Organamerailic
Chemistry Ekvier Sequoia SA. Lame Printed
21
in The Nctherlmds
THE PHOTOLYSIS TRIETHYLBORANE
M. V. ENCINA Lnborarorio
AND
Cen~rai
de
OF ACETONE
IN THE
PRESENCE
OF
E. A. Z.ISSI Quinzica,
Unirrersid&d
Tecnica
de1 Estado,
Au.
Ecuador
3467,
Santiago f Chile)
(Received November 13th, 1970; in revised form January 25th, 1971)
SUMMARY The photolysis ofacetonein the presence of triethylborane has been investigated at 20 and 1260. From an analysis of the hydrocarbons produced and the diminution of the rate of carbon monoxide formation, it is concluded that a reaction between
the acetone triplets and triethylborane
takes place and that this reaction produces
ethyl radicals_The values of the rate constant of this reaction were found to be z 1 x 10’ r at 20 and 126O respectively. The small temperature and 3.4x 10li ml-mole-‘-s-’
dependence of these values points to a very low activation energy. This result is similar to that previously found for the biacetyl/triethylborane system. The rates of attack of methyl and ethyl radicals on triethylborane have also been measured_
INTRODUCTION
The vapor phase photoIysis of acetone is now well understood’. At 313OA, most ofthe significant processes occur from the lower triplet, and their rate constants, as well as their pressure dependence, have been measured’. In a previous paper2 we have reported that triethylborane is a reactive quencher of biacetyl triplets, and that this quenching involves the production of ethyl radicals. As-it has been shown that the photolysis of acetone at low temperatures in the presence of triethylborane (TEB) also produces ethyl radicals3, it was decided to investigate whether these radicals are produced by reaction of some excited species or of methyl and acetyl radicals with TEB. Acetone was therefore photolysed with light of wavelength longer than 2900 A in the presence of TEB at different temperatures, the volatile products being anal_ned by gas chromatography. From our results it can be concluded that reactive quenching of acetone triplets by TEB is the main source of ethyl radicals in this system. EXPERIMENTAL
Two Pyrex reaction cells were employed_ One of them (100 ml) was thermostatted by means of a circulating water jacket. The other one (330 ml) was immersed J. organometaf.Chew,
29 (1971)
21-31
22
M. V.
ENCINA,
E. A. LI&iI
in a thermostatted oil bath. A nearly parallel light beam that illuminated the whole reaction cell was employed_ Li&ht from a medium pressure mercury lamp was filtered using two P_yrexglasses (2.mm thick). This cut off all radiation of wavelength shorter than 2900 A. The reactants were measured inside the reaction cell with a differential manometer; The second reactant was always admitted from a higher pressure to improve the mixing. As dead space was very low, this method assured reasonable mixing of the reactants. To measure the quantum yields, duplicate runs were done with pure acetone immediately before or after each run. TEB was an Ethyl Corporation product. Acetone was an analytical grade reagent from Hopkins and Williams. The lack of any absorption in the 4000 A region showed that the acetone contained less than 0.5 x 10d3 moles per cent of biacetyl. Both reagents were handled under high vacuum conditions and were purified by trap-to-trap distillation_ The reaction products were fractionated by low temperature traps into three fractions : (a)The fraction retainedat - lOO=wasnotanalyzed.(b)The fraction retained at liquid nitrogen temperature was analyzed by gas chromatography op an alumina column. It- consisted of ethane, ethylene, propane, n-butane and small amounts of propylene_ (c)The fraction uncondensableat liquid nitrogen temperature was analyzed on molecular sieves. It consisted of methane and carbon monoxide. RESULTS
Under the conditions employed in the present study, TEB does not react when irradiated by itself. Furthermore, dark runs of mixtures of TEB and acetone did not give any volatile product below 140”. No pressure change could be detected under the same conditions. When acetone alone was photolyzed, the only products detected were carbon monoxide, methane and ethane. At 126O the carbon monoxide quantum yield was higher than 4(C,H,)+*#(CH,) (see Table 2). On the other hand, at 107’, @(CO) was lower than r$(C2H6)++$(CH4). At temperatures lower than 30“, only ethane and small amounts of methane were detected_ Results at low temperature When the mixtures of TEB and acetone were photolyzed at 20°, the results shown in Table 1 were obtained_ In Fig. 1 a plot of the total amounts of methyl radicals produced relative to the amount of ethyl radicals is given as a function of the inverse TEB concentration. The rate of methyl radical formation was evaluated from eqn. (1) where the last term accounts for the ethane arising from the methyl radical recombination. RtCH,) = R(=-L) +R(GHs) + CR(GHs)12/2 R(GH,,) (1) The rate of ethyl radical formation was evaluated from eqn. (2) where the last term is a correction to account for the ethane produced by methyl radical recombination RGH,) = 2 R(CI,H,.)+R(C~H,)+R(C~HS)+R(C~H~)J_ Organo&etnl_ Chem, 29 (1971)21-31
[R(C,-H,)]2/4
R(C,H,,)
(2)
PHOTOLYSIS
TABLE
OF ACETONE
23
OF TRIETHnBoRANE
1
EXPERIMENTAL
RBJLTS
Rates are given in lO_” Total
IN THE PRESENCE
CTEBI
AT 20°C
mole-ml-L~s-l.
Concentrations
WW
WX%)
RGW
RGW
0.60 0.70 0.10 1.27 1.21 2.33 1.97 1.57
0.03 0.09 0.0 0.26 0.16 0.20 0.29 0.32 0.34 0.28 0.15 0.35 0.31 0.19 0.27 0.24 0.21 0.30 0.37 0.34 0.28 0.18 0.21 0.42 0.31 0.36
0.11 0.22 0.0 0.31 0.27 0.33 0.37 0.37 0.40 0.35 0.27 0.63 0.63 0.45 0.59 0.52 0.52 0.57 0.71 0.45 0.44 0.35 0.39 0.64 0.60 0.25
are given in 10e6 mole/ml.
RGHm)
COIlCZl.
1.69 2.44 3.27 3.34 3.58 4.12 4.15 4.15 4.23 4.55 5.16 5.3 1 5.31 5.31 5.31 5.3 1 5.31 5.3 1 5.3 1 5.3 1 5.31 5.3 1 5.31 5.31 5.31 5.3 1
0.83 0.84 0.0 0.85 0.29 0.86 0.81 0.85 0.85 1.23 1.66 0.22 0.22 0.23 0.25 0.27 0.28 0.42 0.48 0.58 0.94 1.39 _I.47 1.59 -1.59 1.59
0.22 0.22 _ 0.08 0.19 0.23 0.12 0.27 0.25 0.23 0.24
LIZ
1.19 0.26 0.21 0.23 0.26 0.24 0.24 0.31 0.22 0.14 0.23 0.22 0.2i
3.05 1.70 1.96 2.46 1.81 I .33 1.96 1.99 1.48 201 2.13 2.39 2.07 2.39 2.74 3.24
0.31 0.21 0.45 -.___-
0.2 i ITEE
Fig. 1. R(CH,)/R(C,H,) 5.31 x lO-6 mole/ml.
as a function
0.0
1.25
1.50 1.97 I .93
2.10
1.32 1.93
0.4 (107ml~mole)
of the inverse TEB concentration.
Temp. 20”. Total concentration
In those runs in which butane was not measured its rate of formation was evaluated from the rate for ethylene. Resrrlts at 107 and 126O The experimental results obtained at 126” are given in Table 2, where some of the data obtained at 107O have also been included. J. 0r@li10meta1. ChtwL 29 (1971)21-31
M. V.
24 TABLE
E. A. LISSI
2
BXL_RSULls
AT TEHP~TUFCES
Rates are given in 10-12 mole- ml-’ Total conca
ENCINA,
[TEB]
HIGHER THAN 100”
-s-l_
Concentrations
are given in low6 mole/d.
NC@
R(C&)
R(C&)
-R(CJ-U
R(C,Hs)
R(C&o)
0.0 0.0
0.0 0.0 1.18 0.53 0.44 0.26
0.0 0.0 0.34 0.15 0.21 0.21
0.0
0.0
0.0
0.0
0.0
0.0
0.41 0.49 0.33 0.30 0.33
0.25 0.08
0.13 0.12 0.09
0.08 0.15 0.04
2.5 2.5 28 3.2 3.72 3.88
1070 0.0 0.0 0.27 0.64 i.22 1.33
6.68 7.10 5.8 4.8 3.6 3.9
1.16 1.01 4.0 3.8 4.2 4.1
8.5 9.0 3.3 4.6 3.1 4.0
3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 320 3.90
126’ 0.0 0.0 0.0 0.23 0.25 0.31 0.34 0.37 0.50 0.67 0.86 1.02 1.08 I.13 0.0 1.17
4.50 4.50 4.47 3.57 3.32 2.83 2.91 2.99 2.71 259 236 2.19 1.64 1.24 6.11 283
1.77 241 2.94 4.44 4.05 4.06 4.02 5.33 3.43. 4.11 3.26 3.03 3.00 2.42 2.71 4.99
2.83
l/TEB (lo7 Fig. 2. ~(CO)/[I&,(CO)+(CO)] centration 3.02 x10-” molehnl.
2.36 1.30 1.12 3.43 1.43 7.84 1.59
274 a33
0.16 0.08 0.12
0.0 0.0 0.0 0.07 0.08 0.05 0.11 .0.09 0.13 0.12 0.12
0.11 0.21
ml/mole)
as a function
of inverse TEB concentration.
Temp.
1260. Total
con-
In Fig_ 2 a plot of b(CO) divided by 4,(CO)-&(CO) [where &(CO) is the carbon monoxide quantum yield iu the photolysis of pure acetone] is given as a function of the inverse TEB concentration. J_ Orgat?ometuZ_Ckm.,
29 (1971) 21-31
PHOTOLYSIS OF ACETONE IN THE PkFSENCE OF TRIETHyLBqRANE
When 1,3-butadiene was added to the acetone instead ofTEB, the diniinution of carbon monoxide was nearly independent of the concentration of 1,3-butadiene (1,Zbutadiene pressures from 5 to 10 mm). Under these conditions WQ)/&(W
-WQ)
= O-6
DISCUSSION
The mechanism of the photolysis of acetone at !ong wavelength in the presence of a reactive quencher can be represented by the set of reactions (3)-(9) where it has A+hv
-
3A*
(3)
3A*
-
CH; + CH&O’
(4)
3A*+M
-
3A+M
(5)
3A+(M)
-
CH;+CH,CO’+(M)
(6)
‘A. 3A
-
A+hv
(7)
-A*
3A+Q
-
(8) Products
(9)
been assumed that almost all the excited molecules cross over to the triplet state. It must he pointed out that if the decomposition arose from the excited singlet this would not modify the following discussion. If TEB is the added quencher and the reactive quenching involves the production of an ethyl radical, the following reactions will occur: 3A+TEB
-
C,H;+P’
C2H;+C,H;
-
C,H,,
(10) (11)
C2H; +C,H:,
-
C2Hs +C2H4
(12)
CH;+CH; C,H;+CH;
-
C2Hs C3Hs
(13) (14)
C2H; +CH;
-
CH,+C2H,
(15)
C,H;+TEB
-
C,H,+p CH4+R’
(16)
-
C,H,+A’
(18)
-
CH4+A
(19)
CH; +TEB C,W5 +A CH; +A
(17)
The formation of propene has been discussed elsewhere4 and is due to a secondary reaction of R-. Low-temperature results The presence of ethyl radicals in the system can be inferred from the hydrocarbons produced and by their relative rates of production as R(C2H4)/R(C4H1e) is nearly 0.13, in agreement with the expected value &om the disproportionation/combination ratio of ethyl radica@. J.
Orgunometal. Gem,
29 (1971) 21-31
26
M. V.
ENCINA,
E. A. LISI
That the ethyl radicals are mainly produced by reaction (10) and not by reaction (20) or by. the acetyl radicals (either in a bimolecular reaction or thorugh the CH;+TEB
-
C,H;+CH,BEt,
(20)
formation of biacetyl and its subsequent reaction with TEB’) can be deduced from the following facts : (a). The Cp(C,H,) obtained at 20” with 15 mm of added TEB is nearly one_ On the other hand, the decomposition quantum yield under similar conditions can be estimated from the data of O’Nea16 as being smaller than 0.20 ifthe same effZency & M is assumed for TEB and acetone. (b). The rate of reaction (20) has been measured in the azomethane/TEB system4 and found to be smaller than the rate of reaction (17). As in the present system eqn. (21) holds, we have then that the rate of reaction (20) must be smaller than the Rls +R17 +R,,
= R(CH,)
(21)
rate of methane production. The experimental results show that the relations (22) and (23) hold. This fact shows that reaction (20) cannot be the main source of ethyl radicals in the present system. R(C&)
> 20 R(CH4)
(22)
R(C2H5) > 20 R,,
(23) (c). The number of acetyl radicals produced can be estimated from the rate of methyl radical production. The proposed set of reactions leads to eqn. (24). If the ethyl R(CH,CO’)
= R(CH;) +R,,-, - R(CH;) +R(CH,)
(24) radicals were produced by reactions of the acetyl radicals or of some of its reaction prpducts, relation (25) would hold. On the other hand, eqn. (26) is found at TEB R(C,H;)/R(CH,CO’)
\( 1
(25)
pressures higher than 5 mm. R(C,H;)/R(CH,CO’} > 5 (26) From the preceding discussion it can be concluded that reaction (10) can be considered as the main source of ethyl radicals. Furthermore, the high quantum yield of ethyl radicals shows that this reactive quenching is the main reaction of acetone triplets in the presence of TEE. If reactions (11) to (19) are the main reactions of methyl and ethyl radicals, their rates of production can be evaluated from eqns. (1) and (2). Combination of other radicals present in the system (Pa, R’, A’ or CH,CO-) with methyl and ethyl radicals should make the use of qns_ (1) and (2) questionable. But, as the rates of recombination of these radicals with methyl and ethyl radicals should be quite similar, it can bc shown that eqn. (27) will approximately hold, even when a significant (R,+R,)/R,,
= (R&H;)
+R,,)I(R(C,H;)
-R,,)
(27) fraction of the small radicals are being consumed by reccmbination with higher radicals. The correction by reaction (20) is a minor one and is easily accounted for as k,, has been measured4. J.
Organometal. Chem, 29 (1971) 21-31
27
PHOTOLYSIS OF ACETONE IN THE PRESENCE OF TRIETHYLBORANE
From reactions (3) to (10) relation (28) can be concluded, where M measures
&+Rci R
10
1
1
K,
-&
=w+k,,’
(28)
the total concentration and k6 will be pressure-dependentTo obtain eqn. (28) the same third body efficiency has been assumed for TEB and acetone2*‘. In eqn. (28), k4 is the only rate constant that will depend upon the initial wavelength. Then, if both M and the wavelength are kept constant, (R4+Rs)/Rlo should depend linearly on the inverse TEB concentration. This plot, with (R,+R&Rro evaluated from eqns. (l), (2) and (27) is shown in Fig. 1. From this plot, the following values can be derived kJk,
= 4.5 x lo-’
mole/ml
and klo = 1 x 10” ml-mole-‘-s-’ if the values of kg, k7 and k8 previously reported are employedl. The value found for kJk, is higher than that previously reported’ for the 313OA radiation (1.5 x lo-’ mole/ml). As k4 is extremely sensitive to the initial energy, this discrepancy may reflect a shorter average wavelength absorbed under the present conditions. Further support for the proposed mechanism can be obtained from the effect of total concentration. From the small slope of Fig. 1 and the fact that k8 > k, at 20°, it can be concluded that most of the 1,:ethyl radicals produced when 15 mm of TEB are present, arise from reaction (4). If under these conditions the contribution of reaction (6) to methyl radical formation is neglected, eqn. (28) reduces to eqn. (29).
(29) A plot of R,/R 1o a g ainst l/M Fig. 3. From this plot a value of
at 15 mm constant TEB pressure is shown in
‘k4/k5 = 4.5 x IO-’ mole/ml can also be obtained, in good agreement with that obtained from Fig. 1. High-temperature results It is generahy assumed’ that, working at low intensities and at temperatures higher than 100°, acetyl radicals can be considered as reacting almost exclusively by
l/M
Fig. 3. Effect of total concentration [TEB] 0.85 x 10e6 mole/ml.
(106ml
/mole)
in R(CHj)jR(C2H5)
at .constant TEB concentration. J. Orgnnometd
Chem, 29
Temp. 20a. (1971) 21-31
28
M. V. ENCINA,
E. A. LISSI
reaction (30) provided that there is no reaction of these radicals other than decomposition, combination and. disproportionation. CH,CO= (+M)
-
CH;+CO
(+M)
(30)
At 107” the mass balance of the runs of acetone alone 2 [4(CO)/+(CH;)] < 1 shows that reaction (30) is competing with other reactions of the acetyl radicals. On the other hand, the results obtained at 126” show that under these conditions it can be considered that the acetyl radicals are reacting nearly quantitatively by reaction (30). Under these conditions eqn. (31) can be then assumed. R(C0)
= lx,+&
(31)
If we assume that eqn. (31) also holds when TEB is added, then the proposkd mechanism leads to eqn. (32), where 4(CO) is the quantum yield of carbon monoxide
WO) 4dCO)
-
-#dCOl
k,
. k,+k,+k, k6
k, -M
+ k,+k,+ks. k 10
C
-‘+k,44
-- k,
k,+k,+k, k,
1-&
(32)
when acetone is photolysed in the presence of TEB, and bo(COj is the quantum yield when acetone alone is photolysed at the same total pressure.Eqn. (32) will not hold if a reaction such as (33) is competing with reaction (30). The importance of reaction CH,CO’+TEB
-
Er+CH,COBEt,
(33)
(33) can be evaluated from the elect of total pressure on @(CO) and from a mass balance. The effect oftotal pressure on 4(CO) is shown in Fig. 4, where results obtained at constant light intensity and acetone and TEB pressures are shown as a fimction of total pressure, which was raised by adding different amounts of propane. From this figure it can be conciuded that the effect of total pressure on 4(CO) is very small. This finding is compatible with the proposed mechanism as TEB is only able to react with thermalized triplets and an increase in total pressurewould decrease the amount
w(lO+
mole/ml
1
Fig_ 4. Effect of total concentrationin R(C0) at constant TEB and acetone concentration and light intensity. [TEB] 0.85 x low6 mole/ml R(C0) is given in IO-” mole. Temp. 126”. [Acetone] 0.85 x 10S6 mole/ml; ml-‘-s-‘. Propane added to increase the total concentration. J_ Organometal. Chem., 29 (1971) 21-31
PHOTOLYSIS
OF ACIXONE
29
IN THE PRFSENCE OF TR BXHYLBORAhG
of acetyl radicals produced by reaction (4) but would increase the rate of reaction (6) These two effects would then tend to cancel out. On the other hand, if there is no quenching of acetone triplets by TEB, the rate of acetyl radical production would be very slightly sensitive to the total pressure, as @(CH,CO’) can be considered to be nearly unity under all conditions. Then relation 34 holds and $(CO) should decrease noticeably when the total pressure decrease 4(CO) = k,,‘(k2~+k;~
(34)
- [TEB])
as reaction (30) is in the fall-off region ‘. We can conclude then that most of the
decrease in carbon monoxide production must arise from reaction (10). The importance of reaction (30) can also be evaluated from a mass balance over the acetyl radicals. The number of acetyl radicals that did not react by reaction (30) can be evaluated from eqn. (35). X = R(CH;) + R,,-2
R(C0)
(35)
When the value of X thus obtained is related to the number W ofethyl radicals released for reactions other than reaction (20) it is found that W is much higher than W = R(C,H;)
- Rzo
(36)
X. This fact shows that most of the ethyl radicals must be produced by reaction (10). That reaction (33) can be contributing to the diminution of carbon monoxide production cannot be completely disregarded as 2 4(CO)/4(CHs) in the presence of TEB is slightly lower than that obtained in the photolysis of acetone alone. Under conditions of low absorption, low conversion and constant light intensity, eqn. (37) will hold, where R(C0) is the rate of carbon monqxide production
WO) 4oW) -4(CO) =
R(CO) R,(CO)
. A/A,-
(37)
R(COj
when acetone at concentration A is photolysed in the presence of a given concentration of TEB, and R,(CO) is the rate of production .of carbon monoxide when pure acetone (concentration A,) is photolysed at the same light intensity_ In Fig. 2 a plot of the left halfofeqn. (37) against the inverse TEB concentration is given. The intercept of this plot has been estimated from the data obtained with 1,3-butadiene as quencher. The quasi independence of the @(CO) on the 1,3-butadiene pressure shows that the thermal triplets are being quantitatively quenchedlO_ The value found for 4(CO)/[+o(CO)-@(CO)] can then be equated to [kJ(k,- M)] (k6+ k, + k&k,. The value found (0.60) can be compared with the vaiue previously reported’ (0.37). The difference between these two values is very likely due to a difference in the average wavelength absorbed (see results obtained at 200). If the proposed mechanism is accepted, a value of (ks+k7+k8)/k10
= 5.9 x lOA mole/ml
can be derived from the slope and intercept of Fig. 2. From this value and previously reported datal, it can he shown that k,, = 3.4x 10” ml-mole-‘-s-’ From the preceding discussion of the contribution
of reaction (33) to the
J- Orgamnetd
Chem.. 29
(1971)21-31
54. V. ENCWA,
30
E. A. LJSSI
decrease in carbon monoxide formation, it can be concluded that this value should be considered as an upper limit for k,e. The similarity between the values obtained for k,, at 20 and 1263 points to a very low activation energy for reaction (10) A similar result has been obtained .for the quenching of biacetyi triplets by TEB’. The difference between the rate constant of reaction (10) at 20” and that found for the reaction of biacetyl triplets with TEB at the same temperature (1.5 x 10” ml-male-1-s-‘) can be partly due to differences in the preexponential A factors or to a difference of nearly 1 kcal/mole iu the activation energies. This difference is well inside the estimated experimental errors. Hydrogen abstraction from TEB If the proposed set of reactions accounts for all the sources of methane, ethane, propane and butane, the rates of reactions (16) and (17) can be evaluated as R1,+Rlg
= R(CH,)-00.04R(C,H,)
R16+R18
= R(C,H,)
(38)
and -0.13
R(C,H,,)-
[R(C,H,)]‘/4
R(C,H,,)
(39)
It can then be shown that k
R17fR19
A-(R&j+
k 17
=&+-.
PEW
(kd
A
and Rl6+R18
(41)
A - (4 A+
By plotting the right halves of equations (40) and (41) against [TEB]/A, values of k17/(kL3)* and kl,J(klL)* can be obtained. These values are shown in Table 3. At 20° the intercept is higher than that expected from the low values of k,, and krg at low temperatures. This fact can be due to some contribution to the rates of methane and ethane formation from reactions (42) and (43) respectiveiy : CH;+CH&O-
-
C,H; +CH,CO-
CH4+CH2C0 -
C,H, +CH,CO
(42) (43)
The contributions of these reactions is not significant at high-[TEB]/A, where a rather high percentage of the radicals abstract hydrogen from TEB. Furthermore, TABLE HYDROGEN
Temp.
3 ABSTRACTION
k,,lk,,i
FROM
TRETHYLRORANE
k,dk, ,+
WI
(mole+-ml-+-s-*)
(mole*-ml-*-s-+)
20 107 126
1.05 co.35 11.6t4.0 19*5
O-65&0.3 5.51-2-O 15*5
J_ Organometol. Chenz.,29 (1971) 21-3:
PHOTOLYSIS OF ACETONE IN THE PRESENCE OF TRIETHYLBORANE
31
the method employed to evaluate k,, and k16 would minimize the error .jntroduccd by this extra source of methane and ethane. The values obtained for kl, are in excellent agreement with those obtained in other systems4 (azomethane/TEB, biacetyI/TEB and di-tert-butyl peroxide/TEB). The values reported for k16, even though they show a rather high dispersion, are in agreement with what can be expected from the relative reactivities of methyl and ethyl radicals towards the same substrate l1 . Furthermore, the relative values of k16/k17 support the previously assumed ratio employed in previous publications2*4 (L&,7=0-5). The relatively high values found for kI 6 and k,, can be related to the weakness of the C-H bond CLto the boron atom4S12. REFERENCES 1 H. E. G'NE~L AND C. W. LARSON,J. PhJx Chem., 73 (1969) 1011. 2 E. ABUIX J. GROTEWOLD, E. A. Lxsst AND M. UMASIA, J. Chem. Sot. A, in the press. 3 J. GROTEWOLD ADD E. A. LISSI, Chem. Commun., (1965)21. 4 J.GROTEWOLD, E. A. L~ssrAND J. C. SCAIANO,J. Chem. Sot-. .+i’,in press. 5 J. 0. TERRY AND J. H. FUTRELL,Cm. J. Chenz., 45 (1967) 2327. 6 H. E. O’NEAL A&D C. W. LARSON, J. Phys. Chem., 70 (1966) 2475. 7 P. C. BEADLE,J.H. Ksox, F. PLACIDO ohm K. C. WAUGH, Tran~. Farodqv SOC., 65 (1969) 1571; T. F. THOMAS, C. I.SIJTINAND C. Snn, J. Amer. Chem. Sot., 59 (1967) 5107. 8 C. P. QUINN Ah?) J. KONSTAXTATOS, Trans. Form@ Sot., 65 (1969) 2693; J. G. CALVERTAND J. N. Prrrs, Photochemiwy, Wiley, New York, 1966, p_ 782. 9 H. E. O’NEAL AND S. W. BEMON, J. Chem. Phys., 36 (1962) 2196; J. A. KERR AND J.G. CALVERT. J. P!zyxChem., 69 (1965) 1022. 10 R. E. REBBERT AND P. AUSLOOS,J. Amer. Chem. SM., 87 (1965) 5569. 11 A. F. TROTMAN-DICEENSON AXD G. S. MUNE, Tables of Bimolecular Gas Reactions. NationalBureau of Standards NSRDS-NBS 9, Washington, 1967. 12 J. GROTEWOLD. E. A. LISSI AND J.C. SCAIANO,J. Organometat. Chem., 19 (1969)431.
J. Organometal. Chem., 29 (1971) 21-31
Chemistry Ekvier Sequoia SA. Lame Printed
21
in The Nctherlmds
THE PHOTOLYSIS TRIETHYLBORANE
M. V. ENCINA Lnborarorio
AND
Cen~rai
de
OF ACETONE
IN THE
PRESENCE
OF
E. A. Z.ISSI Quinzica,
Unirrersid&d
Tecnica
de1 Estado,
Au.
Ecuador
3467,
Santiago f Chile)
(Received November 13th, 1970; in revised form January 25th, 1971)
SUMMARY The photolysis ofacetonein the presence of triethylborane has been investigated at 20 and 1260. From an analysis of the hydrocarbons produced and the diminution of the rate of carbon monoxide formation, it is concluded that a reaction between
the acetone triplets and triethylborane
takes place and that this reaction produces
ethyl radicals_The values of the rate constant of this reaction were found to be z 1 x 10’ r at 20 and 126O respectively. The small temperature and 3.4x 10li ml-mole-‘-s-’
dependence of these values points to a very low activation energy. This result is similar to that previously found for the biacetyl/triethylborane system. The rates of attack of methyl and ethyl radicals on triethylborane have also been measured_
INTRODUCTION
The vapor phase photoIysis of acetone is now well understood’. At 313OA, most ofthe significant processes occur from the lower triplet, and their rate constants, as well as their pressure dependence, have been measured’. In a previous paper2 we have reported that triethylborane is a reactive quencher of biacetyl triplets, and that this quenching involves the production of ethyl radicals. As-it has been shown that the photolysis of acetone at low temperatures in the presence of triethylborane (TEB) also produces ethyl radicals3, it was decided to investigate whether these radicals are produced by reaction of some excited species or of methyl and acetyl radicals with TEB. Acetone was therefore photolysed with light of wavelength longer than 2900 A in the presence of TEB at different temperatures, the volatile products being anal_ned by gas chromatography. From our results it can be concluded that reactive quenching of acetone triplets by TEB is the main source of ethyl radicals in this system. EXPERIMENTAL
Two Pyrex reaction cells were employed_ One of them (100 ml) was thermostatted by means of a circulating water jacket. The other one (330 ml) was immersed J. organometaf.Chew,
29 (1971)
21-31
22
M. V.
ENCINA,
E. A. LI&iI
in a thermostatted oil bath. A nearly parallel light beam that illuminated the whole reaction cell was employed_ Li&ht from a medium pressure mercury lamp was filtered using two P_yrexglasses (2.mm thick). This cut off all radiation of wavelength shorter than 2900 A. The reactants were measured inside the reaction cell with a differential manometer; The second reactant was always admitted from a higher pressure to improve the mixing. As dead space was very low, this method assured reasonable mixing of the reactants. To measure the quantum yields, duplicate runs were done with pure acetone immediately before or after each run. TEB was an Ethyl Corporation product. Acetone was an analytical grade reagent from Hopkins and Williams. The lack of any absorption in the 4000 A region showed that the acetone contained less than 0.5 x 10d3 moles per cent of biacetyl. Both reagents were handled under high vacuum conditions and were purified by trap-to-trap distillation_ The reaction products were fractionated by low temperature traps into three fractions : (a)The fraction retainedat - lOO=wasnotanalyzed.(b)The fraction retained at liquid nitrogen temperature was analyzed by gas chromatography op an alumina column. It- consisted of ethane, ethylene, propane, n-butane and small amounts of propylene_ (c)The fraction uncondensableat liquid nitrogen temperature was analyzed on molecular sieves. It consisted of methane and carbon monoxide. RESULTS
Under the conditions employed in the present study, TEB does not react when irradiated by itself. Furthermore, dark runs of mixtures of TEB and acetone did not give any volatile product below 140”. No pressure change could be detected under the same conditions. When acetone alone was photolyzed, the only products detected were carbon monoxide, methane and ethane. At 126O the carbon monoxide quantum yield was higher than 4(C,H,)+*#(CH,) (see Table 2). On the other hand, at 107’, @(CO) was lower than r$(C2H6)++$(CH4). At temperatures lower than 30“, only ethane and small amounts of methane were detected_ Results at low temperature When the mixtures of TEB and acetone were photolyzed at 20°, the results shown in Table 1 were obtained_ In Fig. 1 a plot of the total amounts of methyl radicals produced relative to the amount of ethyl radicals is given as a function of the inverse TEB concentration. The rate of methyl radical formation was evaluated from eqn. (1) where the last term accounts for the ethane arising from the methyl radical recombination. RtCH,) = R(=-L) +R(GHs) + CR(GHs)12/2 R(GH,,) (1) The rate of ethyl radical formation was evaluated from eqn. (2) where the last term is a correction to account for the ethane produced by methyl radical recombination RGH,) = 2 R(CI,H,.)+R(C~H,)+R(C~HS)+R(C~H~)J_ Organo&etnl_ Chem, 29 (1971)21-31
[R(C,-H,)]2/4
R(C,H,,)
(2)
PHOTOLYSIS
TABLE
OF ACETONE
23
OF TRIETHnBoRANE
1
EXPERIMENTAL
RBJLTS
Rates are given in lO_” Total
IN THE PRESENCE
CTEBI
AT 20°C
mole-ml-L~s-l.
Concentrations
WW
WX%)
RGW
RGW
0.60 0.70 0.10 1.27 1.21 2.33 1.97 1.57
0.03 0.09 0.0 0.26 0.16 0.20 0.29 0.32 0.34 0.28 0.15 0.35 0.31 0.19 0.27 0.24 0.21 0.30 0.37 0.34 0.28 0.18 0.21 0.42 0.31 0.36
0.11 0.22 0.0 0.31 0.27 0.33 0.37 0.37 0.40 0.35 0.27 0.63 0.63 0.45 0.59 0.52 0.52 0.57 0.71 0.45 0.44 0.35 0.39 0.64 0.60 0.25
are given in 10e6 mole/ml.
RGHm)
COIlCZl.
1.69 2.44 3.27 3.34 3.58 4.12 4.15 4.15 4.23 4.55 5.16 5.3 1 5.31 5.31 5.31 5.3 1 5.31 5.3 1 5.3 1 5.3 1 5.31 5.3 1 5.31 5.31 5.31 5.3 1
0.83 0.84 0.0 0.85 0.29 0.86 0.81 0.85 0.85 1.23 1.66 0.22 0.22 0.23 0.25 0.27 0.28 0.42 0.48 0.58 0.94 1.39 _I.47 1.59 -1.59 1.59
0.22 0.22 _ 0.08 0.19 0.23 0.12 0.27 0.25 0.23 0.24
LIZ
1.19 0.26 0.21 0.23 0.26 0.24 0.24 0.31 0.22 0.14 0.23 0.22 0.2i
3.05 1.70 1.96 2.46 1.81 I .33 1.96 1.99 1.48 201 2.13 2.39 2.07 2.39 2.74 3.24
0.31 0.21 0.45 -.___-
0.2 i ITEE
Fig. 1. R(CH,)/R(C,H,) 5.31 x lO-6 mole/ml.
as a function
0.0
1.25
1.50 1.97 I .93
2.10
1.32 1.93
0.4 (107ml~mole)
of the inverse TEB concentration.
Temp. 20”. Total concentration
In those runs in which butane was not measured its rate of formation was evaluated from the rate for ethylene. Resrrlts at 107 and 126O The experimental results obtained at 126” are given in Table 2, where some of the data obtained at 107O have also been included. J. 0r@li10meta1. ChtwL 29 (1971)21-31
M. V.
24 TABLE
E. A. LISSI
2
BXL_RSULls
AT TEHP~TUFCES
Rates are given in 10-12 mole- ml-’ Total conca
ENCINA,
[TEB]
HIGHER THAN 100”
-s-l_
Concentrations
are given in low6 mole/d.
NC@
R(C&)
R(C&)
-R(CJ-U
R(C,Hs)
R(C&o)
0.0 0.0
0.0 0.0 1.18 0.53 0.44 0.26
0.0 0.0 0.34 0.15 0.21 0.21
0.0
0.0
0.0
0.0
0.0
0.0
0.41 0.49 0.33 0.30 0.33
0.25 0.08
0.13 0.12 0.09
0.08 0.15 0.04
2.5 2.5 28 3.2 3.72 3.88
1070 0.0 0.0 0.27 0.64 i.22 1.33
6.68 7.10 5.8 4.8 3.6 3.9
1.16 1.01 4.0 3.8 4.2 4.1
8.5 9.0 3.3 4.6 3.1 4.0
3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 3.02 320 3.90
126’ 0.0 0.0 0.0 0.23 0.25 0.31 0.34 0.37 0.50 0.67 0.86 1.02 1.08 I.13 0.0 1.17
4.50 4.50 4.47 3.57 3.32 2.83 2.91 2.99 2.71 259 236 2.19 1.64 1.24 6.11 283
1.77 241 2.94 4.44 4.05 4.06 4.02 5.33 3.43. 4.11 3.26 3.03 3.00 2.42 2.71 4.99
2.83
l/TEB (lo7 Fig. 2. ~(CO)/[I&,(CO)+(CO)] centration 3.02 x10-” molehnl.
2.36 1.30 1.12 3.43 1.43 7.84 1.59
274 a33
0.16 0.08 0.12
0.0 0.0 0.0 0.07 0.08 0.05 0.11 .0.09 0.13 0.12 0.12
0.11 0.21
ml/mole)
as a function
of inverse TEB concentration.
Temp.
1260. Total
con-
In Fig_ 2 a plot of b(CO) divided by 4,(CO)-&(CO) [where &(CO) is the carbon monoxide quantum yield iu the photolysis of pure acetone] is given as a function of the inverse TEB concentration. J_ Orgat?ometuZ_Ckm.,
29 (1971) 21-31
PHOTOLYSIS OF ACETONE IN THE PkFSENCE OF TRIETHyLBqRANE
When 1,3-butadiene was added to the acetone instead ofTEB, the diniinution of carbon monoxide was nearly independent of the concentration of 1,3-butadiene (1,Zbutadiene pressures from 5 to 10 mm). Under these conditions WQ)/&(W
-WQ)
= O-6
DISCUSSION
The mechanism of the photolysis of acetone at !ong wavelength in the presence of a reactive quencher can be represented by the set of reactions (3)-(9) where it has A+hv
-
3A*
(3)
3A*
-
CH; + CH&O’
(4)
3A*+M
-
3A+M
(5)
3A+(M)
-
CH;+CH,CO’+(M)
(6)
‘A. 3A
-
A+hv
(7)
-A*
3A+Q
-
(8) Products
(9)
been assumed that almost all the excited molecules cross over to the triplet state. It must he pointed out that if the decomposition arose from the excited singlet this would not modify the following discussion. If TEB is the added quencher and the reactive quenching involves the production of an ethyl radical, the following reactions will occur: 3A+TEB
-
C,H;+P’
C2H;+C,H;
-
C,H,,
(10) (11)
C2H; +C,H:,
-
C2Hs +C2H4
(12)
CH;+CH; C,H;+CH;
-
C2Hs C3Hs
(13) (14)
C2H; +CH;
-
CH,+C2H,
(15)
C,H;+TEB
-
C,H,+p CH4+R’
(16)
-
C,H,+A’
(18)
-
CH4+A
(19)
CH; +TEB C,W5 +A CH; +A
(17)
The formation of propene has been discussed elsewhere4 and is due to a secondary reaction of R-. Low-temperature results The presence of ethyl radicals in the system can be inferred from the hydrocarbons produced and by their relative rates of production as R(C2H4)/R(C4H1e) is nearly 0.13, in agreement with the expected value &om the disproportionation/combination ratio of ethyl radica@. J.
Orgunometal. Gem,
29 (1971) 21-31
26
M. V.
ENCINA,
E. A. LISI
That the ethyl radicals are mainly produced by reaction (10) and not by reaction (20) or by. the acetyl radicals (either in a bimolecular reaction or thorugh the CH;+TEB
-
C,H;+CH,BEt,
(20)
formation of biacetyl and its subsequent reaction with TEB’) can be deduced from the following facts : (a). The Cp(C,H,) obtained at 20” with 15 mm of added TEB is nearly one_ On the other hand, the decomposition quantum yield under similar conditions can be estimated from the data of O’Nea16 as being smaller than 0.20 ifthe same effZency & M is assumed for TEB and acetone. (b). The rate of reaction (20) has been measured in the azomethane/TEB system4 and found to be smaller than the rate of reaction (17). As in the present system eqn. (21) holds, we have then that the rate of reaction (20) must be smaller than the Rls +R17 +R,,
= R(CH,)
(21)
rate of methane production. The experimental results show that the relations (22) and (23) hold. This fact shows that reaction (20) cannot be the main source of ethyl radicals in the present system. R(C&)
> 20 R(CH4)
(22)
R(C2H5) > 20 R,,
(23) (c). The number of acetyl radicals produced can be estimated from the rate of methyl radical production. The proposed set of reactions leads to eqn. (24). If the ethyl R(CH,CO’)
= R(CH;) +R,,-, - R(CH;) +R(CH,)
(24) radicals were produced by reactions of the acetyl radicals or of some of its reaction prpducts, relation (25) would hold. On the other hand, eqn. (26) is found at TEB R(C,H;)/R(CH,CO’)
\( 1
(25)
pressures higher than 5 mm. R(C,H;)/R(CH,CO’} > 5 (26) From the preceding discussion it can be concluded that reaction (10) can be considered as the main source of ethyl radicals. Furthermore, the high quantum yield of ethyl radicals shows that this reactive quenching is the main reaction of acetone triplets in the presence of TEE. If reactions (11) to (19) are the main reactions of methyl and ethyl radicals, their rates of production can be evaluated from eqns. (1) and (2). Combination of other radicals present in the system (Pa, R’, A’ or CH,CO-) with methyl and ethyl radicals should make the use of qns_ (1) and (2) questionable. But, as the rates of recombination of these radicals with methyl and ethyl radicals should be quite similar, it can bc shown that eqn. (27) will approximately hold, even when a significant (R,+R,)/R,,
= (R&H;)
+R,,)I(R(C,H;)
-R,,)
(27) fraction of the small radicals are being consumed by reccmbination with higher radicals. The correction by reaction (20) is a minor one and is easily accounted for as k,, has been measured4. J.
Organometal. Chem, 29 (1971) 21-31
27
PHOTOLYSIS OF ACETONE IN THE PRESENCE OF TRIETHYLBORANE
From reactions (3) to (10) relation (28) can be concluded, where M measures
&+Rci R
10
1
1
K,
-&
=w+k,,’
(28)
the total concentration and k6 will be pressure-dependentTo obtain eqn. (28) the same third body efficiency has been assumed for TEB and acetone2*‘. In eqn. (28), k4 is the only rate constant that will depend upon the initial wavelength. Then, if both M and the wavelength are kept constant, (R4+Rs)/Rlo should depend linearly on the inverse TEB concentration. This plot, with (R,+R&Rro evaluated from eqns. (l), (2) and (27) is shown in Fig. 1. From this plot, the following values can be derived kJk,
= 4.5 x lo-’
mole/ml
and klo = 1 x 10” ml-mole-‘-s-’ if the values of kg, k7 and k8 previously reported are employedl. The value found for kJk, is higher than that previously reported’ for the 313OA radiation (1.5 x lo-’ mole/ml). As k4 is extremely sensitive to the initial energy, this discrepancy may reflect a shorter average wavelength absorbed under the present conditions. Further support for the proposed mechanism can be obtained from the effect of total concentration. From the small slope of Fig. 1 and the fact that k8 > k, at 20°, it can be concluded that most of the 1,:ethyl radicals produced when 15 mm of TEB are present, arise from reaction (4). If under these conditions the contribution of reaction (6) to methyl radical formation is neglected, eqn. (28) reduces to eqn. (29).
(29) A plot of R,/R 1o a g ainst l/M Fig. 3. From this plot a value of
at 15 mm constant TEB pressure is shown in
‘k4/k5 = 4.5 x IO-’ mole/ml can also be obtained, in good agreement with that obtained from Fig. 1. High-temperature results It is generahy assumed’ that, working at low intensities and at temperatures higher than 100°, acetyl radicals can be considered as reacting almost exclusively by
l/M
Fig. 3. Effect of total concentration [TEB] 0.85 x 10e6 mole/ml.
(106ml
/mole)
in R(CHj)jR(C2H5)
at .constant TEB concentration. J. Orgnnometd
Chem, 29
Temp. 20a. (1971) 21-31
28
M. V. ENCINA,
E. A. LISSI
reaction (30) provided that there is no reaction of these radicals other than decomposition, combination and. disproportionation. CH,CO= (+M)
-
CH;+CO
(+M)
(30)
At 107” the mass balance of the runs of acetone alone 2 [4(CO)/+(CH;)] < 1 shows that reaction (30) is competing with other reactions of the acetyl radicals. On the other hand, the results obtained at 126” show that under these conditions it can be considered that the acetyl radicals are reacting nearly quantitatively by reaction (30). Under these conditions eqn. (31) can be then assumed. R(C0)
= lx,+&
(31)
If we assume that eqn. (31) also holds when TEB is added, then the proposkd mechanism leads to eqn. (32), where 4(CO) is the quantum yield of carbon monoxide
WO) 4dCO)
-
-#dCOl
k,
. k,+k,+k, k6
k, -M
+ k,+k,+ks. k 10
C
-‘+k,44
-- k,
k,+k,+k, k,
1-&
(32)
when acetone is photolysed in the presence of TEB, and bo(COj is the quantum yield when acetone alone is photolysed at the same total pressure.Eqn. (32) will not hold if a reaction such as (33) is competing with reaction (30). The importance of reaction CH,CO’+TEB
-
Er+CH,COBEt,
(33)
(33) can be evaluated from the elect of total pressure on @(CO) and from a mass balance. The effect oftotal pressure on 4(CO) is shown in Fig. 4, where results obtained at constant light intensity and acetone and TEB pressures are shown as a fimction of total pressure, which was raised by adding different amounts of propane. From this figure it can be conciuded that the effect of total pressure on 4(CO) is very small. This finding is compatible with the proposed mechanism as TEB is only able to react with thermalized triplets and an increase in total pressurewould decrease the amount
w(lO+
mole/ml
1
Fig_ 4. Effect of total concentrationin R(C0) at constant TEB and acetone concentration and light intensity. [TEB] 0.85 x low6 mole/ml R(C0) is given in IO-” mole. Temp. 126”. [Acetone] 0.85 x 10S6 mole/ml; ml-‘-s-‘. Propane added to increase the total concentration. J_ Organometal. Chem., 29 (1971) 21-31
PHOTOLYSIS
OF ACIXONE
29
IN THE PRFSENCE OF TR BXHYLBORAhG
of acetyl radicals produced by reaction (4) but would increase the rate of reaction (6) These two effects would then tend to cancel out. On the other hand, if there is no quenching of acetone triplets by TEB, the rate of acetyl radical production would be very slightly sensitive to the total pressure, as @(CH,CO’) can be considered to be nearly unity under all conditions. Then relation 34 holds and $(CO) should decrease noticeably when the total pressure decrease 4(CO) = k,,‘(k2~+k;~
(34)
- [TEB])
as reaction (30) is in the fall-off region ‘. We can conclude then that most of the
decrease in carbon monoxide production must arise from reaction (10). The importance of reaction (30) can also be evaluated from a mass balance over the acetyl radicals. The number of acetyl radicals that did not react by reaction (30) can be evaluated from eqn. (35). X = R(CH;) + R,,-2
R(C0)
(35)
When the value of X thus obtained is related to the number W ofethyl radicals released for reactions other than reaction (20) it is found that W is much higher than W = R(C,H;)
- Rzo
(36)
X. This fact shows that most of the ethyl radicals must be produced by reaction (10). That reaction (33) can be contributing to the diminution of carbon monoxide production cannot be completely disregarded as 2 4(CO)/4(CHs) in the presence of TEB is slightly lower than that obtained in the photolysis of acetone alone. Under conditions of low absorption, low conversion and constant light intensity, eqn. (37) will hold, where R(C0) is the rate of carbon monqxide production
WO) 4oW) -4(CO) =
R(CO) R,(CO)
. A/A,-
(37)
R(COj
when acetone at concentration A is photolysed in the presence of a given concentration of TEB, and R,(CO) is the rate of production .of carbon monoxide when pure acetone (concentration A,) is photolysed at the same light intensity_ In Fig. 2 a plot of the left halfofeqn. (37) against the inverse TEB concentration is given. The intercept of this plot has been estimated from the data obtained with 1,3-butadiene as quencher. The quasi independence of the @(CO) on the 1,3-butadiene pressure shows that the thermal triplets are being quantitatively quenchedlO_ The value found for 4(CO)/[+o(CO)-@(CO)] can then be equated to [kJ(k,- M)] (k6+ k, + k&k,. The value found (0.60) can be compared with the vaiue previously reported’ (0.37). The difference between these two values is very likely due to a difference in the average wavelength absorbed (see results obtained at 200). If the proposed mechanism is accepted, a value of (ks+k7+k8)/k10
= 5.9 x lOA mole/ml
can be derived from the slope and intercept of Fig. 2. From this value and previously reported datal, it can he shown that k,, = 3.4x 10” ml-mole-‘-s-’ From the preceding discussion of the contribution
of reaction (33) to the
J- Orgamnetd
Chem.. 29
(1971)21-31
54. V. ENCWA,
30
E. A. LJSSI
decrease in carbon monoxide formation, it can be concluded that this value should be considered as an upper limit for k,e. The similarity between the values obtained for k,, at 20 and 1263 points to a very low activation energy for reaction (10) A similar result has been obtained .for the quenching of biacetyi triplets by TEB’. The difference between the rate constant of reaction (10) at 20” and that found for the reaction of biacetyl triplets with TEB at the same temperature (1.5 x 10” ml-male-1-s-‘) can be partly due to differences in the preexponential A factors or to a difference of nearly 1 kcal/mole iu the activation energies. This difference is well inside the estimated experimental errors. Hydrogen abstraction from TEB If the proposed set of reactions accounts for all the sources of methane, ethane, propane and butane, the rates of reactions (16) and (17) can be evaluated as R1,+Rlg
= R(CH,)-00.04R(C,H,)
R16+R18
= R(C,H,)
(38)
and -0.13
R(C,H,,)-
[R(C,H,)]‘/4
R(C,H,,)
(39)
It can then be shown that k
R17fR19
A-(R&j+
k 17
=&+-.
PEW
(kd
A
and Rl6+R18
(41)
A - (4 A+
By plotting the right halves of equations (40) and (41) against [TEB]/A, values of k17/(kL3)* and kl,J(klL)* can be obtained. These values are shown in Table 3. At 20° the intercept is higher than that expected from the low values of k,, and krg at low temperatures. This fact can be due to some contribution to the rates of methane and ethane formation from reactions (42) and (43) respectiveiy : CH;+CH&O-
-
C,H; +CH,CO-
CH4+CH2C0 -
C,H, +CH,CO
(42) (43)
The contributions of these reactions is not significant at high-[TEB]/A, where a rather high percentage of the radicals abstract hydrogen from TEB. Furthermore, TABLE HYDROGEN
Temp.
3 ABSTRACTION
k,,lk,,i
FROM
TRETHYLRORANE
k,dk, ,+
WI
(mole+-ml-+-s-*)
(mole*-ml-*-s-+)
20 107 126
1.05 co.35 11.6t4.0 19*5
O-65&0.3 5.51-2-O 15*5
J_ Organometol. Chenz.,29 (1971) 21-3:
PHOTOLYSIS OF ACETONE IN THE PRESENCE OF TRIETHYLBORANE
31
the method employed to evaluate k,, and k16 would minimize the error .jntroduccd by this extra source of methane and ethane. The values obtained for kl, are in excellent agreement with those obtained in other systems4 (azomethane/TEB, biacetyI/TEB and di-tert-butyl peroxide/TEB). The values reported for k16, even though they show a rather high dispersion, are in agreement with what can be expected from the relative reactivities of methyl and ethyl radicals towards the same substrate l1 . Furthermore, the relative values of k16/k17 support the previously assumed ratio employed in previous publications2*4 (L&,7=0-5). The relatively high values found for kI 6 and k,, can be related to the weakness of the C-H bond CLto the boron atom4S12. REFERENCES 1 H. E. G'NE~L AND C. W. LARSON,J. PhJx Chem., 73 (1969) 1011. 2 E. ABUIX J. GROTEWOLD, E. A. Lxsst AND M. UMASIA, J. Chem. Sot. A, in the press. 3 J. GROTEWOLD ADD E. A. LISSI, Chem. Commun., (1965)21. 4 J.GROTEWOLD, E. A. L~ssrAND J. C. SCAIANO,J. Chem. Sot-. .+i’,in press. 5 J. 0. TERRY AND J. H. FUTRELL,Cm. J. Chenz., 45 (1967) 2327. 6 H. E. O’NEAL A&D C. W. LARSON, J. Phys. Chem., 70 (1966) 2475. 7 P. C. BEADLE,J.H. Ksox, F. PLACIDO ohm K. C. WAUGH, Tran~. Farodqv SOC., 65 (1969) 1571; T. F. THOMAS, C. I.SIJTINAND C. Snn, J. Amer. Chem. Sot., 59 (1967) 5107. 8 C. P. QUINN Ah?) J. KONSTAXTATOS, Trans. Form@ Sot., 65 (1969) 2693; J. G. CALVERTAND J. N. Prrrs, Photochemiwy, Wiley, New York, 1966, p_ 782. 9 H. E. O’NEAL AND S. W. BEMON, J. Chem. Phys., 36 (1962) 2196; J. A. KERR AND J.G. CALVERT. J. P!zyxChem., 69 (1965) 1022. 10 R. E. REBBERT AND P. AUSLOOS,J. Amer. Chem. SM., 87 (1965) 5569. 11 A. F. TROTMAN-DICEENSON AXD G. S. MUNE, Tables of Bimolecular Gas Reactions. NationalBureau of Standards NSRDS-NBS 9, Washington, 1967. 12 J. GROTEWOLD. E. A. LISSI AND J.C. SCAIANO,J. Organometat. Chem., 19 (1969)431.
J. Organometal. Chem., 29 (1971) 21-31